Not directly a paleo issue, but interesting:

http://www.scienceblog.com/cms/low-carb-diets-affect-cognition-skills-17983.html

For my own part, I start my day eating a lot of fruits, so no
carb-deficit early in the day.  But having read this, I'm going
to pay more attention to my attention-span in the afternoons
and experiment with snacking on more fruits to combat any
cognitive fatigue later in the day.

:j